The invention relates to a light-diffractive binary grating structure and the use thereof in a security element as set forth in the classifying portions of claims 1 and 10.
Such grating structures are distinguished by a high level of diffraction deficiency and pronounced asymmetry and polarisation of the diffracted light and are suitable for the surface patterns which are known as visual security features, comprising a mosaic of surface elements occupied by diffractive structures.
Those security features serve as an authenticity feature which is difficult to forge, for valuable articles, documents, identity cards or passes, bank notes and so forth and are described for example in EP 0 105 099 B1. The surface elements of the security features have periodic diffractive grating structures with predetermined grating parameters such as profile, spatial frequency and azimuthal orientation and so forth, in such a way that the aspect of the illuminated security feature changes in a predetermined fashion in respect of color and pattern, upon being rotated and/or tilted. If oppositely oriented asymmetric grating structures are arranged in adjacent surface elements, as is known for example from EP 0 360 969 A1, the levels of intensity of the surface elements, such levels being perceived by a viewer, are interchanged when the substrate is turned through 180°.
DE 32 06 062 A1 shows a simple binary grating structure and the use thereof for a security feature. The binary grating structure has as its profile a structure which follows a simple periodic rectangular function. The pulse duty cycle of the rectangular function, that is to say the ratio of the width of the valley or trough to the length of the period of the rectangular structure, is constant and is of a value of 50% so that the diffracted light has the greatest degree of color saturation.
A development of the security feature with binary grating structures is described in EP 0 712 012 A1. A surface element of the security feature is occupied by a periodic rectangular structure. The pulse duty cycle of the rectangular function continuously changes over the surface element from a maximum value to a minimum value so that, when the surface element is illuminated, the degree of color saturation reaches a maximum in a region within the surface element. Also shown are non-periodic rectangular structures which are arranged with randomly selected dimensions in respect of depth and width to form a diffraction structure in order to enhance the anti-forgery aspect of the security elements. It will be noted however that the resulting pattern is highly complex and is difficult for an observer to identify as the genuine security feature.
U.S. Pat. No. 4,846,552 discloses an electron beam lithography process for producing binary grating structures for focusing optical elements. Even with the simple rectangular structures with a pulse duty cycle of 50%, it is possible to achieve high levels of diffraction efficiency for linearly polarised light by a clever choice of the depth of the structure, for example 25% of the period length of the rectangular structure. The importance of the binary grating structures is based on the fact that they can be produced with a high degree of accuracy and replicated by molding.
The above-mentioned simple rectangular structures can be copied using holographic means, in which respect, in the case of a copy of the symmetrical rectangular structure with a pulse duty cycle of about 50%, there is only a slight drop in the level of diffraction efficiency. The level of diffraction efficiency of the copy worsens, the more asymmetrical that the rectangular structure is. A surface element with the rectangular structure whose pulse duty cycle continuously changes along a predetermined direction in accordance with EP 0 712 012 A1 has in the copy, instead of the changing color saturation, a variation in the intensity of the diffracted color. As changing color saturation is visually difficult to distinguish from changing color intensity without reference to a genuine security element, an unauthorised copy of the security element cannot be immediately recognised.